For a model of a backyard tool shed that is 12′ x 16′ … after installing the ceiling joists I stopped and wondered if the instalation was correct. Because of the un-equal wall heights the joists seem to get nailed off in mid air to the longer rafters. It makes me wonder if the joists are enough for triangulation, or should I make the ceiling joists the bottom chord of a truss?
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Replies
That design uses "rafter
That design uses "rafter ties". It's a compromise -- gives you more headroom but a little less stable roof. Lots of older (ca 1920-1950) garages were built with this technique, and a goodly number of "story and a half" homes.
The main problem is that the rafters will tend to bow over time. But on a rafter of maybe only 10 feet long it's not a big deal, so long as the rafters are sized large enough. A less common problem, but more serious, is that the ties can pull loose (especially given the common practice back then of only putting a tie every 2-4 rafters). But that's just a matter of proper fastening (and putting a tie on every rafter).
In your asymetric setup, if you lower the ties and tie off to the studs on the other side (to keep the ties horizontal) you'll cause the studs to bow, so no gain doing that. You could run the ties diagonally I suppose, but that's probably overthinking it.
Adding a king post or otherwise adding more wood above the tie (at its current height) will not do anything to improve strength -- the rafter will still want to bow.
I'd leave well enough alone, so long as the rafters are not undersized. (Many such garages were built with 2x6 rafters and some with 2x4s. It looks from the drawing like you have 2x8 rafters, which should be plenty heavy enough.)
We were typing at the same time
You got to the "post" button sooner than I though...
Collar Ties for Everyone!
DanH,
Thanks for your oppinion. I had felt like I made half ceiling joist and half collar tie ... that's what set me to thinking that these ceiling joists were just not adequate to triangulate the roof. The only thing I could come with after that was to move the joists inboard and turn them into bottom chords of a truss. So, I did that ... then I moved that building to the base of the Rocky Mountains and set it on the road right next to the rock slide.
Are you saying that I should add collar ties to each rafter? The rafters and ceiling joists are 2x8's. The slope of the roof is 9:12. The walls are 2x6's. The length of the ceiling joists is 112" The floor joists are 2x8's and the sub-floor/ finished floor is 2x12 planks ... all sitting on 6x6 rails. The ridge board is 9.0625" which is the plumb depth of a 2x8 rafter for 9:12 ... so, for a back yard tool shed it's plenty strong. The birdsmouth just makes it leaving a plumb distance of 4.9375" of relish to carry the weight of the roof. And with the exception of the ceiling joists, that may be the other weak spot, in that, I cut the birdsmouth at 6.25" to allow .75" plywood sheathing to slip behind the rafter tail.
So, collar ties everyone? OK
Are you saying that I should add collar ties to each rafter?
No, what you have (closer to rafter tie than collar tie) is plenty strong so long as the rafters are stiff enough. You don't need collar ties (which would be higher up) nor king posts nor anything else.
Well, that is kinda the nature of the salt box design shed....
As long as the long rafters are made with large enough material you will be fine. Just make sure the CJs are well nailed. Like maybe 5 or 6 nails at each end.
The first thing I ever built was a salt box shed. 40 years later and it is still standing. Designed by my dad. I guess he wanted to keep his 14 YO son out of trouble. Look what happened. He shoulda bought me some interesting law books or medical journals.
Also - in your drawing it looks like you have ridge posts - studs supporting the ridge - almost as if the ridge is a "ridge beam" rather than a ridge board. That will help too. In summary if you have 2x6 rafters with a 2x8 ridge you will have a stout structure - assuming you don't have a 100 PSF snow load in your area....
Lost My Reply!
Matt,
I had just finished replying to your post and tried to leave an attachment when I clicked on the wrong file and in an attempt to get back lost the entire post!
All it said was that I was impressed with your 'eye' that you could see that there may be a post or something holding up the ridge board. What's happening is a happy accident. I was leaning so heavy towards installing a truss system, that I actually did. Afterward, thinking that I may have jumped the gun, I made a second copy of the drawing and put the ridge board back. Because the rafters are gussetted together at the ridge line, in my partially restored drawing, my ridge board was now backed with a peak. So, after removing the peak, I found myself squaring the rake wall top plates! Happy accident. In real life, I'm not that smart! In fact, until you pointed it out, I hadn't realized the value of what I had done.
I began using SketchUp because of my inabillity to visualize. But I instinctually knew that if I could draw it, I could certainly build it. And, where I'm self-taught, SketchUp and Breaktime have been filling the gaps in my education. I love 'em both.
So, the rafters are 2x8's, and the ridge is 9.0625" which is = to the plumb cut of the rafters which have a slope of 9:12. And I want to thank-you for pointing out that the ridge will have a bearing of 5.5" at each end wall. Thank-You!
So, you think I'll be fine ... that the structure is strong enough. No collar ties?
No collar ties.
I really don't think they are needed on something that small. And with 2x8 rafters and ceiling joists it will be anything but underbuilt. Maybe a bit overbuilt. I think you said something about Colorado, so I guess you could have quite a snow load so I'd say you are good. OTOH the 2x8 ceiling joists are a little over the top. You see in your application the ceiling joists will have little to no load other than being in tention.
Also, I may not be understanding your usage of the word "truss", but then I think you used the word gusset so maybe not. I'm not seeing that home made trusses would do much for this situation but add more work.... Especially with the sized lumber you are lookiing at using. Generally trusses are used so that smaller lumber (and shorter too) can span wider spans. Your span is snall.
Many Happy Retrurns
Hi Matt,
You and DanH were a big help ... in the end I decided on a hybrid construction where I kept the ridge beam and gusseted the ceiling joists to the rafters. Then I moved on to try and figure out the boxed returns. I had seen a hip roof used for the boxed returns on the PBS series "The New Yankee Workshop" about a Garden Shed w/Recycling Center and liked the idea. It took me all day to figure it out for my little salt box, but in the end we are all happy.
This is just an update
CubeSquare